It is proposed to continue the study of the pathogenesis of experimental liver cancer with emphasis on two aspects - (a) the presence, nature and modulation of DNA damage and its repair during various phases of liver cancer induction, with increasing concentration on enzymatic mechanisms; and (b) the nature and properties of a new antigen, PN antigen, associated with preneoplastic and neoplastic hepatocyte populations and the role of the new antigen in pathogenesis. Increasing attention will also be placed on the relation of PN antigen to interrupted liver cell differentiation.